In direct-current arc furnaces the bottom electrode is the most highly stressed component. In the simplest case it consists of a steel body which extends through the vessel bottom and its lining and makes the electrical contact with the melt in the interior of the furnace. The bottom electrode is connected to the current supply of the furnace outside the vessel bottom.
When the furnace is in operation, the bottom electrode melts to a certain depth. This melt mixes with that of the furnace. The depth to which the bottom electrode melts off depends on various factors:
on the magnitude of the Joule effect, which is produced by the passage of current through the bottom electrode;
on the heat flow from the melt to the bottom electrode;
on the degree of cooling of the bottom electrodes.
In order to ensure reliable operation of the furnace, the consumption of the bottom contact must be restricted. The heating-up due to the Joule effect can be controlled by dimensioning the bottom electrode for a suitable current density. In contrast, the heat flow from the melt to the bottom electrode is of a complex nature, two mechanisms being superimposed:
The furnace current passing through the melt and the molten part of the bottom contact leads to a bath agitation (electromagnetic stirring), which promotes heat exchange. Superimposed on this bath agitation is a process which in the relevant literature is referred to as chemical boiling and which is caused by the liberation of carbon monoxide from the molten part of the bottom contact. This "boiling", which occurs in particular in melts into which oxygen is blown in order to reduce the carbon content, leads to an intensive heat exchange with the bottom electrode and may cause the consumption of the bottom electrode to below the permissible value.
By means of a suitable cross-section of the bottom electrode and cooling measures, the bath agitation due to electromagnetic interactions can be controlled and thus the consumption restricted. Up to now the only measure to restrict the damaging effects of chemical boiling consists in dividing up the bottom electrode into a multiplicity of thin metal rods, which are all embedded in refractory material of the vessel bottom (cf. EP-A-0,058,817). The bridges made of refractory material which remain on consumption of the metal rods hinder the bath agitation in the region of the bottom electrode and consequently prevent excessive consumption of the metal rods. Bottom electrodes of this type are, however, very uneconomical in particular for large arc furnaces.